KABUL, Afghanistan (CNN) --Six children have been found crushed to death by a collapsed wall after a U.S. assault on a compound in Afghanistan, the U.S. military says.

Lt. Col. Bryan Hilferty said Wednesday it was not clear whether the children, along with two adults also killed, died as a direct result of the raid on Friday in the city of Gardez.

A day later, nine children died following a U.S. airstrike in the central Afghan village of Petaw, about 100 miles (160 kilometers) southwest of Kabul.

"After we went in there (the Gardez compound), we determined the next day ... the bodies of two adults and six children (were found) under a collapsed wall," Hilferty said at Bagram Air Base.

"We don't know what caused the wall to collapse because, although we fired on the compound, there were secondary and tertiary explosions."

The complex housed a huge cache of weapons of renegade Afghan commander Mullah Jalani, but he was not at the site at the time of the attack, Hifferty said.

Several other people were arrested after the assault by warplanes and troops, he added, but did not say whether the two adults killed were combatants or civilians.

The U.S. military -- which last weekend launched "Operation Avalanche", its biggest operation against militants since the fall of the Taliban two years ago -- says it found a huge cache of weapons at the compound including hidden storage compartments containing hundreds of rockets, mortars, rocket-propelled grenades, anti-tank and anti-personnel mines and several howitzers

The graves of nine children killed in a U.S. airstrike on Saturday.

Details of Friday's incident follow a U.S. military blunder in the central Afghan village of Petaw on Saturday when nine children and a man were found dead after an attack by an A-10 ground attack aircraft.

The plane was targeting a Taliban suspect. Though U.S. officials initially said former Taliban district commander Mullah Wazir was killed in the attack, U.S. ambassador Zalmay Khalilzad on Tuesday said they were no longer certain.

Villagers say a local laborer was the man killed in the assault.

U.S. officials have apologized for the incident in Ghazni, which sparked outrage and concern both locally and abroad.

U.N. Secretary-General Kofi Annan, who said he was "profoundly saddened" by the deaths and urged a full investigation.

The U.S. is probing the attack to clarify what happened to the children.

Operation Avalanche is focusing on areas where international troops and aid workers have been hit by terrorist cells, the U.S. military says.

Over 11,000 American troops involved in the offensive have been trying to track down remnants of the Taliban and al Qaeda sympathizers in eastern and southern Afghanistan.

But the militants have stepped up attacks in recent months, targeting foreign aid workers and perceived allies of the U.S.-led coalition.

On Saturday at least 15 people were wounded after a bomb attached to a bicycle exploded in Kandahar's main square.